linux-kernel - Re: [PATCH] Remove needless flush_dcache

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Message-ID: <20090116061657.GC6515@barrios-desktop>
Date:	Fri, 16 Jan 2009 15:16:57 +0900
From:	MinChan Kim <minchan.kim@...il.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Matthew Wilcox <matthew@....cx>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] Remove needless flush_dcache_page call

On Fri, Jan 16, 2009 at 07:13:41AM +0100, Nick Piggin wrote:
> On Fri, Jan 16, 2009 at 03:08:30PM +0900, MinChan Kim wrote:
> > On Thu, Jan 15, 2009 at 10:57:30PM -0700, Matthew Wilcox wrote:
> > > Most I/O devices will do DMA to the page in question and thus the kernel
> > > hasn't written to it and the CPU won't have the data in cache.  For the
> > > few devices which can't do DMA, it's the responsibility of the device
> > > driver to call flush_dcache_page() (or some other flushing primitive).
> > 
> > Hmm.. Now I am confusing. 
> > If devicer driver or with DMA makes sure cache consistency,
> > Why filesystem code have to handle it ?
> 
> Because the filesystem is accessing the page directly rathe rthan going to
> IO.
> 
> Basically, whoever reads or writes the page is responsible to avoid user
> aliases. You see these calls in the VM for anonymous pages, in bounce
> buffer layers, in filesystems that read or write from pages that are
> exposed to userspace (ie. metadata generally need not be flushed because
> it will not be mmapped by userspace).

Totally, understand.
Thanks for kind answering to my poor question in patience.
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